1. Field of the Invention
The present invention relates to an element substrate that discharges liquid by applying discharge energy to the liquid, and to a liquid discharging head including the element substrate.
2. Description of the Related Art
A liquid discharging head that discharges liquid is required to discharge small droplets having a volume of 2 pl or less. By applying such small droplets onto a printing medium at high density, a high-definition image can be obtained. With downsizing of the droplets, the number of discharges dramatically increases. When increasing the number of discharges, it has a limitation to simply increase the discharge frequency. Further, as the discharge frequency increases, the discharging speed sometimes decreases. To prevent the decrease in discharging speed and to discharge a predetermined amount of liquid in a shorter time, there has been adopted an element substrate in which multiple discharge ports are arranged at high density.
The element substrate that discharges liquid has a problem of the increase in viscosity of liquid due to the decrease in temperature of the liquid. To suppress such a problem, a method is adopted which heats liquid before supplying the liquid to action chambers that apply discharge energy to the liquid. However, in the element substrate that discharges small droplets, there is found a problem in that the discharge characteristics are deteriorated by the increase in viscosity with the increase in temperature of the liquid. That is, heated liquid evaporates via the discharge ports even when it stays in the action chambers. In the element substrate that discharges small droplets, the amount of liquid to be discharged from each discharge port is small. Hence, even when a small amount of solvent evaporates, the viscosity of the liquid easily increases. Further, since the discharge ports and the action chambers are relatively small, the liquid is likely to be affected by the increase in flow resistance due to the increase in viscosity. Such a problem is pronounced particularly in pigment ink that is likely to aggregate and high-function ink having a high content of additive resin.
The increase in flow resistance deteriorates the discharge characteristics of the element substrate. When the discharge characteristics are deteriorated, the element substrate is sometimes brought into a state in which it cannot discharge the liquid unless a recovery process is performed for liquid supply channels that reach the discharge ports and the action chambers.
U.S. Patent Application Publication No. 2009/160896 A1 discloses an element substrate that is controlled so that liquid in action chambers is not heated more than necessary.
In the element substrate described in the above publication, heating resistance elements serving as energy generating elements preheat the liquid in the action chambers to a predetermined temperature, and then discharge the liquid by boiling the liquid. However, it is difficult for the heat amount of preheating to rapidly heat the liquid. For this reason, in a low-temperature environment, a long standby time is required to heat the liquid to the predetermined temperature by preheating. This decreases the throughput of the element substrate.
In this way, the element substrate disclosed in the above publication cannot evenly cope with the increase in viscosity of the liquid within the usual use range. Further, to cope with the increase in viscosity of the liquid, it has been proposed to add heating elements different from the heat generating elements in the action chambers and to heat the liquid in the action chambers by the heating elements only by the necessary amount when needed. In such a case in which the heating elements are mounted in the substrate, most heat generated from the heating elements is transferred to the substrate, and this lowers the efficiency. Further, since the heat transferred to the substrate is stored in a liquid discharging head, it may continue heating the liquid even after the heating elements stop heating. For this reason, the heating state continues for a long time, and more liquid than necessary sometimes evaporates.